The present invention relates to electrostatic painting of surfaces and, more particularly, to an improved apparatus and method for such painting which causes the paint to be more efficiently focussed and applied onto the surfaces.
Conventional charge injectors include a needle located at and projecting from the spray end of a rotating paint nozzle, which has a bulbous shape. As the nozzle rotates, the paint is ejected generally tangentially to the curvature of the nozzle and across the electrode which charges the droplets. This causes the droplets to be thus spread over a wide area of the surface to be painted. The shape of the envelope surface formed by the taper of the charging electrode within the paint flow conventionally is that of the standard Pierce electrode as described, for example, in "Applied Charged Particle Optics, Part C: Very-High-Density Beams" edited by A. Septier, Academic Press, 1983, pp. 141 et seq. and pp. 207 et seq. It results in paint droplets having a charge to mass ratio of only about 0.0004 C/g (Coulombs per gram), or one extra electron for about every billion atoms. By contrast, advanced electrostatic precipitators deposit ten times this amount on 0.1 micrometer sized fly ash particles; but even this is an order of magnitude less than the theoretical limit set by electrical breakdown of the air around a particle.
While such electrodes produce acceptable results, it is desirable that the costs thereof be reduced and that the painting be made more efficient. In the manufacture of motor vehicles, these desires stem from a need to produce a more competitively priced product. Further, there is a need to devise better methods for facilitating easier compliance with the EPA (Environmental Protection Agency) requirements for allowed quantity of volatile material released per square foot of surface painted.